UNI-T UT71E calibration question

[Lightages]

Hello, first post but long time electronics dabbler here.

I recently purchased a Unitrend UT71E meter and I suspect that it is not as close to accurate as it could be. It seems to be within spec, but right at the limits. After seeing the youtube videos on someone calibrating the meter using a built in routine and a Fluke reference I see that the values they keep entering for calibration are multiples of 19. Are these multiples of 19 needed or is it just needed to have values at X.X000 or XX.000 or X00.00 for examples?

Any help would be appreciated.

[Kiriakos-GR]

but right at the limits

The limits gets always effected , even by the temperature of the environment that you live.
For instance the winter-summer  difference in temperature it can cause different readings.

As about the question in what voltages the calibration should be made,
there is profile-forms for its instrument , and the one who owns the calibrator ,
follows the directions of the profile-form of the specific instrument.

    

[saturation]

Its common for DMMs to use the maximum input for a full scale reading during calibration.  For example, to calibrate the 2.0000 scale, the input voltage required is 2.0000.  But it depends on whatever the maker wants, the Fluke 87V uses mainly 6: 600mV, 6V, 60V etc., while the Agilent 1252a uses multiples of 5 for full scale performance tests, but 3 for calibration.  I don't know specifically what the UT71E requires, but chances are that's it. 

If you're not sure of your DMMs accuracy, you can do a performance test.  You'll measure the maximum input for a particular range to see that it meets the published specifications.  For an example, download the Agilent 1252a or Fluke 87V service manual and just what they consider a performance check.

[Lightages]

It does look like they use multiples of 19 for almost all of the calibrations, and in OHMs and ACV too. I have an email waiting for reply from UNI-T on this. Maybe they will reply with useful information. MY meter seems to be a low on every range I can compare with my other meters by around 0.4%. Now I would not normally trust old meters against a new one but when three meters I have all say that the UT71E is reading 0.4% low then I do need to question the claim of 0.025% basic accuracy and 0.05% accuracy on the ranges I am using (ignoring the +5 digits). The reason I purchased this meter was the claimed accuracy and expected it to not be a order of magnitude out of spec. My expectation was that it would be with 0.1% accuracy at least.

Now I suppose that all my other meters have drifted all equally off the mark by almost exactly the same amount even though they are different designs and years, but it seems highly implausible.

Now the question is; Where do I get this thing re-calibrated? It is not easy to find such a service here as I am currently living in Chile. Perhaps I can build a reference for the voltages of 1.9V and 19V easily enough but for 190V and 700V I am getting into expensive territory. A high voltage OP amp and precision resistors might get me to 190V but at what accuracy?

[saturation]

Sorry to hear Lightages.  One issue with equipment makers with a less established reputation, is that they overstate [ or lie!] about specs since the sheet is the basis for comparison against other DMMs and its price.  Only a review by a 3rd party, like our sysop Dave started and others have posted on eevblog, brings it to light.  Maybe its just a quality control issue, and they don't calibrate them properly before shipping them out, but regardless sloppy engineering simply doing a calibration could mean other things are equally haphazard.  

https://www.eevblog.com/forum/index.php?topic=533.msg7695#msg7695

To recalibrate it yourself without the right gear, you need the calibration sequence from the service manual.  To do this on the cheap by yourself, you also need at least one good calibrated DMM that is equal to or greater in accuracy than the meter you are going to calibrate.  You also need adjustable AC, DC and ohms sources that are stable for even just a few minutes so you can compare the calibrated DMM against the device under test.

Digital DMM can be a pain to calibrate without the right equipment because you need to set the cardinal value precisely by what the meter wants, you can't input 1.901, it must be precisely 1.900 or the meter will be incorrectly set.  Also the values cannot be any other value, you can't substitute a stable 1.700 for 1.900 because the meter is going to store that value in a set of tables to use to offset measurements by.  In analog instruments, you can set the calibration literally to anything you want, preferrably as close to, but not necessarily the exact value, specified in the manual.

[tekfan]

WOW! That's one helluva Fluke DC calibrator! Hear those relays click! Music to my ears!

[Mechatrommer]

...around 0.4%. ..to question the claim of 0.025% basic accuracy and 0.05% accuracy on the ranges. My expectation was that it would be with 0.1% accuracy at least.
...build a reference for the voltages of 1.9V and 19V easily enough but for 190V and 700V I am getting into expensive territory.

your concern about accuracy means you are embracing expensive territory. 0.4% at 19V is what? ±0.076V so your dmm read 18.924V? for that big number i usually just round off to 19V. at most electronics say 5V, err = ±0.02V 3.3V err = ±0.0132V

as saturation said. in addition, it maybe they are not lying, its just their interpretation on how to get the claimed figure. they maybe choosing the max accuracy dmm (instead of averaged or mod) from their testing and they probably have large standard deviation, meaning you get more chance of getting the lemon (out of spec) from your purchase.

i never characterize my UT71A properly, since its reading very close/comply to my other $20 DMM, so i'm happy. i dont really care much if its .1 out in 1st decimal, and i believe i'm blessed if the error is in 2nd decimal place. because i know... accuracy % = reciprocal $

[Lightages]

Yes I am concerned about accuracy. I have been all my conscious life. I am managing a number of solar power installations and knowing voltages accurately is necessary for the health of the batteries. The comparison of readings below is a mixture of real world values I need to monitor mixed in with other AA cell values and some stacked 9V battery values. It is obvious that the UT71E is reading low compared to my other meters.

UT71E :                       83.82          54.01          44.91            35.428             13.558          9.18         3.0786             1.5136
Reference meter #1:    84.2            54.2            45.1              35.6                 13.61            9.22         3.09                 1.52
Reference meter #2:    84.2            54.1            45.0              35.49               13.60            9.18         3.110               1.527

I use a battery monitor that measures a Kelvin shunt to determine the state of charge and discharge of the batteries. The accuracy of the setup of the monitor is crucial to the accurate monitoring of the state of the batteries. Large deep cycle batteries used in these situations are VERY expensive and the small changes in voltages that relate to their state of being requires equipment that can read them accurately.

I know, why did I take a chance on anything other than the big names? Well mainly because of the relatively favorable results I saw on the EEVblog and other reviews. The price also seemed right and not ridiculously cheap at the same time.

[Bored@Work]

Now the question is; Where do I get this thing re-calibrated? It is not easy to find such a service here as I am currently living in Chile.

Ask

Code: [Select]

DICTUC S.A. National Custodian Laboratory
Vicuna Mackenna # 4860 - Macul
Casilla 306 (correo22)
Santiago
if they do UNI-T's or know someone. If they do, expect to pay a bunch of money for a calibration.

[Kiriakos-GR]

Well , there is always the option to build something like this .. 

https://www.eevblog.com/forum/index.php?topic=1148.0

[Lightages]

Thanks for the information. I will try to call them. Usually my searches for anything in Chile result in 100s of results pointing to web portals of useless information.

[Mechatrommer]

Code: [Select]

DICTUC S.A. National Custodian Laboratory
Vicuna Mackenna # 4860 - Macul
Casilla 306 (correo22)
Santiagoif they do UNI-T's or know someone. If they do, expect to pay a bunch of money for a calibration.

i think Fluke DMM is a cheaper option.

[saturation]

Eyeballing this values, it seems the Uni-T is consistently off ~ 0,4%, so its either just in need of a calibration or the reference meters are off.  An important thing is the Uni-T has not proven its long term stability, so even after you calibrate it can drift into inaccuracy shorter than the claimed calibration period or it will drift with environmental conditions up or down, averaging to the proper value when the conditions are similar to the calibration conditions.  This will give you erratic readings on the same test point over time that is climate dependent, very annoying.

A simple check for the kind of precision and accuracy you need is to get a voltage reference to make a spot check.  Since they are far more accurate than your meters, even if they drift in 1 year, it will still check the state of your meters, even if limited to just one range.

https://www.eevblog.com/forum/index.php?topic=650.45

See this thread, both Malone and Geller have the accuracy you need.  Simpler still if you can find someone nearby in Chile with a known calibrated DMM and check those meters against it.

Yes I am concerned about accuracy. I have been all my conscious life. I am managing a number of solar power installations and knowing voltages accurately is necessary for the health of the batteries. The comparison of readings below is a mixture of real world values I need to monitor mixed in with other AA cell values and some stacked 9V battery values. It is obvious that the UT71E is reading low compared to my other meters.

UT71E :                       83.82          54.01          44.91            35.428             13.558          9.18         3.0786             1.5136
Reference meter #1:    84.2            54.2            45.1              35.6                 13.61            9.22         3.09                 1.52
Reference meter #2:    84.2            54.1            45.0              35.49               13.60            9.18         3.110               1.527

I use a battery monitor that measures a Kelvin shunt to determine the state of charge and discharge of the batteries. The accuracy of the setup of the monitor is crucial to the accurate monitoring of the state of the batteries. Large deep cycle batteries used in these situations are VERY expensive and the small changes in voltages that relate to their state of being requires equipment that can read them accurately.

I know, why did I take a chance on anything other than the big names? Well mainly because of the relatively favorable results I saw on the EEVblog and other reviews. The price also seemed right and not ridiculously cheap at the same time.

[saturation]

The problem I find mecha, see the links on the post of slburris,

https://www.eevblog.com/forum/index.php?topic=533.msg7695#msg7695

is that on that thread and on this link, Uni-T overstates its accuracy.  If they quote their statistics as you say, that is still misleading.  At least Fluke, Agilent, HP, B&K, and all other reputable makers I know either understate their specs not exaggerate them, so you know what you are getting.  If it was calbrated properly but drifted, or they are quoting statistics improperly, its still not excusable.  Further, those are new machines, not years old from calibration date.

But you are right, real accuracy, that not only is spot on but stable over time, is not cheap, but I can say with great confidence of you buy a known reputable make like Fluke or Agilent it will be what the spec sheet says it is; many really old Fluke 80 series DMM are still in spec, decades since it was last calibrated [ I have 4 of them].

Accuracy is important when measuring electrical phenomena related to natural events: physiology, chemistry, etc., because in many instances the thresholds are truly finite, not relative.  The cutoff voltage for NiMH cells is ~ 1.0 Vdc and to go lower risks damage to the cell.  OTAH, deep discharging them close to this is a good way to prolong their service life, so the margin for error for good performance is narrow.

If your DUT provides 1.01V, meter should be 10x more accurate or measure to 0.001 or 0.1% to resolve 0.01V unambiguously.  For example at 0.5% 1.01V becomes 1.005-1.015 and is more uncertain compared to 0.1% which provides 1.011-1.008.

...around 0.4%. ..to question the claim of 0.025% basic accuracy and 0.05% accuracy on the ranges. My expectation was that it would be with 0.1% accuracy at least.
...build a reference for the voltages of 1.9V and 19V easily enough but for 190V and 700V I am getting into expensive territory.

your concern about accuracy means you are embracing expensive territory. 0.4% at 19V is what? ±0.076V so your dmm read 18.924V? for that big number i usually just round off to 19V. at most electronics say 5V, err = ±0.02V 3.3V err = ±0.0132V

as saturation said. in addition, it maybe they are not lying, its just their interpretation on how to get the claimed figure. they maybe choosing the max accuracy dmm (instead of averaged or mod) from their testing and they probably have large standard deviation, meaning you get more chance of getting the lemon (out of spec) from your purchase.

i never characterize my UT71A properly, since its reading very close/comply to my other $20 DMM, so i'm happy. i dont really care much if its .1 out in 1st decimal, and i believe i'm blessed if the error is in 2nd decimal place. because i know... accuracy % = reciprocal $

[Mechatrommer]

cant you open it up and try to find any potential trim pot to adjust, resistor devider etc etc.  if it broke while tinkering, maybe a good excuse to get a Fluke just a 2cnt suggestion.

[Lightages]

cant you open it up and try to find any potential trim pot to adjust, resistor devider etc etc.  if it broke while tinkering, maybe a good excuse to get a Fluke just a 2cnt suggestion.

There might be a trim pot, or maybe ten, but what does it do and which one is correct? If I do find the right adjustment pot, how do I calibrate the meter without a standard 10x better than the meter, never mind anything the same accuracy. There has to be a specific procedure such as described in the video that should be used and used with a good calibration source.

The point is I purchased something as being a certain specification with a one year guarantee that should be close to spec if not right on, not an order of magnitude out.

My emal has been answered by Unitrend, but so far they are only asking what country I am in. More to follow as I get more information from them

[saturation]

Alas, from the video it looks like close case calibration, which is done by inputting values into memory locations.  That's how most modern DMM calibrate these days; old style is with trim pots [ which is better for DIY calibration!].

You may want to see this new coincidental post that is relevant to our topic, but it shows what you pay for with Fluke and Agilent:

https://www.eevblog.com/forum/index.php?topic=3050.msg40602#new

[Mechatrommer]

cant you open it up and try to find any potential trim pot to adjust, resistor devider etc etc.  if it broke while tinkering, maybe a good excuse to get a Fluke just a 2cnt suggestion.

...how do I calibrate the meter without a standard...

there are, you just mentioned it, in the beginning of this thread (2nd post?). thats how you know your 71E reads lower.

[Lightages]

cant you open it up and try to find any potential trim pot to adjust, resistor devider etc etc.  if it broke while tinkering, maybe a good excuse to get a Fluke just a 2cnt suggestion.

...how do I calibrate the meter without a standard...

there are, you just mentioned it, in the beginning of this thread (2nd post?). thats how you know your 71E reads lower.

I suspect my meter is lower. I do not have proof but rather very convincing evidence. I purchased a meter to gain accuracy, not just merely repeat the values I can rwad with my 20 year old meter. So I await the response from Unitrend. Perhaps they will pay to have it calibrated in Chile as their specs are guaranteed for a year.

[Lightages]

You may want to see this new coincidental post that is relevant to our topic, but it shows what you pay for with Fluke and Agilent:

https://www.eevblog.com/forum/index.php?topic=3050.msg40602#new

I have read that post and it does show the quality control on the Unitrends are not all that great. But when I see Dave's reviews on multimeters the Unitrends seem to be OK in his accuracy tests. I might be naive, but when a big company states a certain spec and then also provides a year guarantee to cover it, I expect the specs to be close, not an order of magnitude out.

[alm]

Dave only tested a single sample of each, you need lots of samples to get any meaningful statistics about quality control. Dave also didn't test long-term stability (takes lots of time, obviously).

I might be naive, but when a big company states a certain spec and then also provides a year guarantee to cover it, I expect the specs to be close, not an order of magnitude out.

Big companies are just as likely to produce crap as small ones. You might expect quality brand products to easily meet their specs, but Uni-T is not at all in the same class as the likes of Agilent, Fluke and Gossen. There also seem to be fewer (no?) complaints about the stability/accuracy of brands like B+K Precision and Extech, although this may be because they're less popular.

[Lightages]

For those of you still interested, I have a reply from UniTrend on how to calibrate the meter.

"Thanks for your feetback very much,I  want to know the real reason of the "lower" value in order to how prevent from it happen again,but very sorry,I can't open the video of your website.
I notice that there are some precise calibration source in the picture of your website,so I believe that you should be able to calibrate the UT71E if you know how to adjust it.
I send the calibrating istruction for UT71E as an attachment ,For details, pls see the appendix.
Thanks
Flyhone Tsang"

I have attached the file so that it can be of use to others. If adjusting the basic level trimpot gets rid of the offset, then I will be completely happy with the meter. I will be buying a voltage reference as I probably should have one anyway.

And I have have now included the file in Open Office or Libre Office format for those of you who prefer it in that format.

[saturation]

Amazingly simple; it has a trimmer and close case calibration?  This calibrates all ranges, including Vac, ohms etc.,?  If just Vdc that's adequate, but if all ranges, that's impressive.

Studying this, thanks for the files will report later.  What most important is if you have limited access to test gear in Chile, being able to calibrate DIY in your own lab gives you a way to compensate for the Uni-Ts faults.  Even before you get the voltage references, you could adjust the Uni-T to match your trusted meters and see how it holds across the range.

For those of you still interested, I have a reply from UniTrend on how to calibrate the meter.

"Thanks for your feetback very much,I  want to know the real reason of the "lower" value in order to how prevent from it happen again,but very sorry,I can't open the video of your website.
I notice that there are some precise calibration source in the picture of your website,so I believe that you should be able to calibrate the UT71E if you know how to adjust it.
I send the calibrating istruction for UT71E as an attachment ,For details, pls see the appendix.
Thanks
Flyhone Tsang"

I have attached the file so that it can be of use to others. If adjusting the basic level trimpot gets rid of the offset, then I will be completely happy with the meter. I will be buying a voltage reference as I probably should have one anyway.

And I have have now included the file in Open Office or Libre Office format for those of you who prefer it in that format.

[Mechatrommer]

great job! thanx for sharing.

[alm]

Amazingly simple; it has a trimmer and close case calibration?  This calibrates all ranges, including Vac, ohms etc.,?  If just Vdc that's adequate, but if all ranges, that's impressive.

Looks like they have no ability to adjust VAC, resistance or current, these are just performance checks (although they don't actually document these, only VDC is mentioned).

Even before you get the voltage references, you could adjust the Uni-T to match your trusted meters and see how it holds across the range.

This does appear to require a stable source with the exact values (within accuracy specs / 5 or so) required.